DIY replacement of the exhaust – Rumbling noises require immediate action!

If the car is getting noisy whilst driving quality remains the same, the exhaust is often the issue. Due to its simple construction, the mostly cheap materials and its uncomplicated installation, its replacement is no problem even for laymen. Read here what to observe when replacing an exhaust.

The exhaust is one of the most strained car parts, constructed as a wear part in order not to make the car too expensive. This means the exhaust has a limited life span.

The waste gas flow line

On their way into the open air, waste gases pass the following stations:
exhaust manifold
y-pipe
flex pipe
catalytic converter
centre pipe
middle silencer
end silencer
tail piece

• Every combustion in the engine produces waste gases, flowing via the outlet valve past the manifold gasket into the manifold. The manifold is a curved pipe, leading the hot outflow along the vehicle’s underbody. The manifold is attached to the engine and therefore considerably affected by vibrations.

  It is built as a particularly heavy and massive cast steel component. The manifold generally lasts a car’s life span. In case of serious unbalance in the engine it can become cracked. It is one of the more expensive components of the exhaust, although it can be installed as a used part. Yet, there is no rule without exception: in some cars the catalytic converter is integrated in the manifold.

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• Connected to the manifold, the y-pipe brings the exhaust flow from the separate combustion chambers together in one duct. This component is quite massive as well. The lambda sensor is integrated in the manifold. Its task is measuring the remaining oxygen in the exhaust flow and transmitting these data to the control unit. The y-pipe can be installed as a used part as well.

• The y-pipe is followed by the short flex pipe. This component, measuring just a few inches, is the exact opposite of the heavy and massive cast-steel manifold and y-pipe when it comes to construction. Consisting of stainless steel fabric, it is very flexible and can easily move in all directions. There’s a good reason for this: the flex pipe absorbs the strong vibrations from the engine, preventing them from effecting the subsequent components.

• The flex pipe is followed by the catalytic converter. This component cleans the exhaust. It is essential for this component to remain unaffected by the engine’s vibrations. Its ceramic internal component would otherwise break.

• Following the catalytic converter comes the actual exhaust pipe which is often equipped with a middle silencer. Since 2014 another sensor is standard installed in the centre pipe, measuring the catalyst’s functioning. This sensor is referred to as the diagnostic sensor.

• The end silencer is connected to the centre pipe. This is where the actual noise reduction takes place. The end silencer ends in the tail piece. The entire exhaust is mounted to the underbody of the car with simple, though very massive rubbers. They keep the piping at an even distance from the bottom of the car. Simultaneously they allow a swing, preventing the rigid pipe from bending.

Weak points of the exhaust

• The most strained component of the exhaust is the flex pipe. It must endure extreme temperature swings and is constantly wrung. Nevertheless, this 15 EUR (± £13) component is astonishingly durable. If it develops cracks, this is noticed immediately as the engine produces a deafening noise. With a cracked flex pipe, even a 45 HP car soon sounds like a Formula 1 race car.

• The end silencer is most affected by defects. This component consists of thin, galvanised steel plating. Not only is it exposed to extreme temperature swings. During its cooling phase the exhaust attracts condensation.

  In the end silencer the moisture mixes with the exhaust soot, forming a slightly acidic fluid which corrodes the exhaust pipe from the inside out. From the other side, rust, stimulated by road salt, eats its way through the end silencer’s plating.

  An end silencer therefore lasts only several years. A defective end silencer is identified by the engine noises gradually becoming louder. At visual inspection, black smudges can be found on the component. These are the points where the waste gas escapes, leaving a trace of soot.

• The catalytic converter announces its malfunction by rattling and knocking noises, indicating the broken ceramic core. The pieces roll around in the housing. Sooner or later the noises will cease – the housing is empty. The entire core has crumbled to dust and is blown out by the exhaust flow.

  This will ultimately show at the next MOT inspection: a car without catalytic converter will not pass the emission test. With help of the recently standard installed diagnostic sensors, this defect is noticed much sooner.

No fear of the defective exhaust

The exhaust is one of the easiest parts to repair. Prices for the separate components however, differ greatly. The most expensive part by far is the catalytic converter, which can cost more than 1000 EUR (± £900).

You could try replacing it by a used part, however you never know if a used catalytic converter is still functioning properly.

Flex pipe, middle silencer and end silencer are considerably cheaper and can be purchased separately. Especially the end silencer, depending on quality and driving style can be “bust” after a few years. In most cases this is no problem at all.

A new end silencer for most car series costs less than 100 EUR (± £90). The same applies to the middle silencer. The middle pipe is amazingly robust in most cars. Although it does not last as long as the manifold or the y-pipe, it is not a specific wear part.

Repairing the exhaust system

In the technical sense, the exhaust consists of a set of joined pipes, held together by clamps. Theoretically they can be easily separated. In praxis, rust and dirt often cause the pipes to cake together. Before you pull until your fingers bleed, applying an angle grinder is the best way. Always see to it that sparks fly away from the car. Ideally the underbody is covered when grinding off an old exhaust. Nevertheless, be very careful: the sparks constitute a high risk of fire!

If grinding cannot be avoided, always work sensibly: only remove the defective part. The intact part should be left undamaged. It makes no sense cutting up a catalytic converter in order to remove a flex pipe. Instead, the remaining piece can be removed from the old part with a screwdriver and a couple of hammer blows.

No use welding

Welding the exhaust pipe makes no sense. Even in new condition, the metal is so thin that it can hardly be properly welded. If the end silencer is full of holes, virtually no sufficiently strong plating is left. The complete replacement of the silencer is quicker, cleaner and more durable than welded patches.

Full replacement is the easiest way

As an alternative to replacement of single defective components it is evident to replace the entire exhaust. “Entire” means everything except catalytic converter, including flex pipe.
Dismantling and removing the old piping is considerably easier. Furthermore, a completely new exhaust offers maximum safety and life span. Equal strain on all components causes them to wear simultaneously.

If the flex pipe ruptures, a corroded end silencer will follow soon. The cheap prices for complete exhaust systems (without catalytic converter) make complete replacement of all wear parts particularly easy. Exhaust replacement always includes replacement of the rubbers. Porous exhaust rubbers will be criticised at MOT inspection.
This can be avoided at little expense. Complete exhaust systems without catalytic converter are available at less than 100 EUR (± £90), depending on car model.

Foto: Dmitry Kalinovsky, kvsan, KUNANEK SUPAKOSOL, Apinunt Sukhapinda, homydesign, Einar Muoni, fabiodevilla, Besjunior, Visionsi, Aleksey Rodionov / shutterstock.com